1. Field of the Invention
The present invention relates to drying devices, and more particularly to a drying device adapted for improved and faster and more comfortable drying of a user's hands and/or hair.
2. Description of the Related Art
Conventional hand dryers dry an individual's wet hands in one of two ways, evaporative drying or “blow-off” drying. (In the blow-off case, a small amount of evaporation occurs, but it is incidental and minimal since the airstream is not warmed.) Conventional evaporative hand dryers include a blower for generating an air stream through a ducting system to an exit air outlet that directs the air stream onto the hands of the user. The air stream is heated by a heating device to evaporate the moisture off the user's hands. The hand dryers generally include a push button, sensor or other means to actuate the blower and heater for a predetermined time period (e.g., 30 seconds).
The drying time for conventional evaporative hand dryers is relatively slow, taking 30 to 45 seconds or more to dry a user's hands. Conventional dryers suffer from low energy efficiency. The low energy efficiency is a result of the following operating factors: heating up the internal dryer components; not maximizing and optimizing air flow temperature, direction and velocity; not compensating locally for evaporative cooling; and not addressing the problem of a stagnation boundary layer of air and water molecules which inhibits evaporation of water at the skin surface of the hands. Attempts to improve energy efficiency in the prior art include providing an enclosure for the hands, recirculating air and predrying the air.
A major impediment to evaporation is the presence of a stagnation boundary layer, which is a region adjacent to the surface of the water. The stagnation boundary layer corresponds to the transition region from where air containing evaporated water molecules are moving and where water molecules adjacent to the water surface (or any other surface) are not moving or moving much slower. In this stagnation boundary layer, the water molecules evaporating will accumulate, and about as many will flow back to the water surface as will flow away into the flowing stream of air. This stagnation boundary layer inhibits the net evaporation of surface water. By breaking up the stagnation boundary layer with a strong component of air flow perpendicular to the surface, the evaporation is increased. Rather than accumulating in the stagnation boundary layer and inhibiting the net evaporation of water, the water molecules in the stagnation boundary layer are swept away, as fast as they accumulate, by the air breaking up the stagnation boundary layer. U.S. Pat. No. 6,038,786, the entire contents of which are incorporated herein by reference, discloses a hand dryer that improves dispersion of the boundary layer.
To diffuse the stagnation boundary layer, a second type of conventional hand dryers uses “blow-off” or “air knife” technology instead of evaporation (although a small amount of evaporation occurs). These blow-off dyers provide an intensive blast of high velocity air which when suitably deployed, blows or skives droplets of water off the user's hands.
It has been found that after using a conventional “blow-off” hand dryer, the hands feel cold and slightly moist, as a result of the heat loss and subsequent cooling due to evaporation of some of the residual moisture that has not been blown off. The hands are cooled during blow off drying because even air that has not been heated will evaporate some water, and the remaining water and surface will thus be cooled by the heat loss due to evaporation. This discomfort is present during drying and for about 30 seconds after drying until the hands return to normal temperature.